Hitherto, in the microwave and millimeter wave signal processing arts, the well known diode pair-hydrid coupled double balanced mixer has been used to combine an incoming radio frequency (RF) signal with a local oscillator (LO) signal in order to obtain a desired output intermediate frequency (IF) signal for further signal processing using conventional state-of-the-art IF mixers, amplifiers, detectors and the like. For example, using such a balanced mixer circuit, an RF signal of 94 GHz and a local oscillator signal of 93 GHz can both be initially coupled through a 3db coupler into a single waveguide, then channeled to individual mixer diodes of reverse polarity, and then, finally, recombined to generate an IF difference frequency signal of, for example 1 GHz for further amplification and signal processing. By using two separate diodes instead of the single diode that is often used in single ended mixers (such as the single ended mixer disclosed in U.S. Pat. No. 4,006,425 assigned to the present assignee), the local oscillator power requirement per mixer diode is substantially reduced.
Whereas the two diode balanced mixers of the type described generally above have proven satisfactory for certain kinds of millimeter wave applications, the conventional balanced mixer circuit construction most frequently used for these mixers requires that the local oscillator signal frequency be quite close to the frequency of the incoming RF signals. Thus, this requirement places a certain power output demand on the local oscillator used and therefore makes it impossible, for example, for a 20 milliwatt Gunn oscillator to drive more than about 2 balanced mixers. It is desirable in certain monopulse radar applications to have a single local oscillator drive many more than two mixer diodes to enable a single local oscillator to maintain proper phase and amplitude tracking of sum and difference IF signals. Thus, the above prior art balanced mixer circuit construction is unsuitable for such applications.
Therefore, in order to provide a mixer circuit wherein a single local oscillator is capable of driving many mixer diodes and a corresponding large plurality of antennae in certain types of monopulse radar applications, it has been previously proposed to suspend two parallel-connected mixer diodes of reverse polarity in a waveguide structure and then apply local oscillator and RF signals, respectively to opposite ends of the waveguide in order to produce differential currents in the diodes. The electromagnetic fields produced by these differential currents cause the second harmonic of the LO frequency to mix with the RF incoming signal frequency and thereby reduce by one-half the required local oscillator frequency. Since local oscillators operating at lower frequencies produce much more output power than those operating at higher frequencies, a single low frequency oscillator can now drive many mixer diodes for monopulse radar applications. Therefore, using the latter approach, a local oscillator operating at 46.5 GHz can now be used at one end of the waveguide to mix its second harmonic frequency of 93 GHz with the 94 GHz RF input signal frequency and still provide the required plus or minus 1 GHz IF output signal. Such harmonic mixing has been disclosed, for example, by Joseph M. Baird et al in U.S. Application Ser. No. 864,027, entitled "Bi-mode Millimeter Wave Mixer", and assigned to the present assignee.
The above type of diode pair suspended-in-waveguide structure has also been described in at least two other configurations. One of these configurations utilizes two reverse poled diodes connected in parallel between opposite walls of a conventional rectangular waveguide. The other configuration utilizes a pair of reverse poled diodes connected to a suspended metal plane of either a stripline or dipole structure, and this latter structure is in turn suspended in a larger waveguide structure. The first of the above suspended-in-waveguide configurations is described by M. V. Schneider and W. W. Snell in an article entitled "Stripline Down Converter With Subharmonic Pump", Bell System Technical Journal, Vol. 53, pages 1179-1183, July-August 1974. This first configuration is also disclosed in another article by the same authors entitled "Harmonically Pumped Stripline Down Converter", IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-23, pages 271-275, March 1975. The other type of suspended-in-waveguide diode pair mixer configuration is described by M. Cohn, J. E. Degerford, and B. A. Newman in IEEE Trans. Microwave Theory Tech., Vol. Mtt-24, pp. 254-257, May 1976, in an article entitled "Harmonic Mixing With Antiparallel Diode Pair".
Whereas these suspended-in-waveguide mixers do in fact reduce the frequency and power requirements for the local oscillator, they nevertheless are relatively expensive in design, and are generally too large and unsuitable for use at millimeter wave frequencies greater than about 100 GHz.